The correct answer is endoplasmic reticulum.
Any nucleated cell contains MHC class I proteins which present cytosolic peptides. For example, during the intracellular microorganism infection such proteins are degraded in the proteasome, then degraded peptides are internalized in the endoplasmic reticulum and there associated with MHC-I molecules which are freshly synthesized. MHC-I/peptide complex is displayed on the cell surface and ready to interact with T lymphocytes.